Recently, there is a need to recycle used catalysts in order to reduce waste generation. In particular, the performance of an exhaust gas denitration catalyst for exhaust gas of a boiler using coal or heavy oil as fuel is degraded over time by alkali metals, alkaline-earth metals, and arsenic compounds. Most of such poisonous substances (e.g., heavy metals) attached to the denitration catalyst can be dissolved in a cleaning liquid in the cleaning process to remove the poisonous substances.
A conventional method for cleaning used catalysts is generally carried out by immersing used catalysts in the form of unit in a cleaning liquid tank having a size that can accommodate one or more units. In the cleaning, water, sulfuric acid, oxalic acid, ammonia, sulfate, and various other cleaning liquids are generally used singly or in combination. The denitration performance of the catalysts is known to be recovered by such cleaning (e.g., Patent Documents 1 and 2). Moreover, during cleaning, it is reportedly effective to stir the cleaning liquid with air bubbles (aeration), or generate cavitation by ultrasonic vibration (e.g., Patent Document 3). This is because poisoning components and impurities can be removed in a short time in the cleaning process. The cleaned catalyst is immediately dried, and gas generated by aeration or cavitation in the cleaning process is generally vented to the atmosphere as it is.